Jack Gardner, museum chairman and great-great-nephew of the founder, said the new addition is expected to cost $60 million and possibly more but will triple its room for special exhibitions.

"When Mrs. Gardner created the museum and its original endowment, she couldn't possibly have envisioned the challenges the museum would face in a 100 years from the perspective of conservation and public duties, Gardner said.

Isabella Gardner, a New York department store heiress who married a Boston socialite, built the museum in the style of a Venetian palazzo from 1899 to 1901 to house her collection of more than 2,500 artworks including paintings by Sandro Botticelli, Raphael, Jan Vermeer, Rembrandt, Edgar Degas, and John Singer Sargent. It has been open to the public since 1903.

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